1. Field of the Invention
The present invention relates to a lithium ion polymer secondary battery composed of a lamination of a positive-electrode sheet, a negative-electrode sheet, and a polymer electrolyte layer provided therebetween, and to a gelatinous polymer electrolyte for sheet batteries which is used in the polymer electrolyte layer.
2. Description of the Related Art
In recent years, thin batteries have been in increasing demand with the spread of portable devices, such as video cameras and notebook personal computers. A typical thin battery is a lithium ion polymer secondary battery which is formed by laminating a positive-electrode sheet and a negative-electrode sheet. The positive-electrode sheet is prepared by forming an active material on a surface of a positive-electrode collector foil, and the negative-electrode sheet is prepared by forming an active material on a surface of a negative-electrode collector foil. A polymer electrolyte layer is disposed between the active material on the positive-electrode sheet and the active material on the negative-electrode sheet. The positive-electrode collector foil and the negative-electrode collector foil are provided with a positive-electrode terminal and a negative-electrode terminal, respectively, from which a current generated by a potential between these two active materials is extracted. Such a laminate is hermetically packed to form a lithium ion polymer secondary battery. The positive-electrode terminal and the negative-electrode terminal are led out from the package and are used as terminals to supply a predetermined potential from the lithium ion polymer secondary battery.
Polymer solid electrolytes using ion-conducting polymers have been used as electrolytes for such sheet batteries in order to prevent leakage of electrolyte solutions. In polymer solid electrolytes, electrolytes are homogeneously dissolved into polymers. The polymer solid electrolytes are flexible and are suitable for use in sheet batteries. A problem of the polymer solid electrolytes is significantly low ion conductivity compared to electrolyte solutions. Thus, batteries using the polymer solid electrolytes exhibit low charging current densities and high electrical resistance.
In order to solve such a problem, Japanese Unexamined Patent Publication No. 10-321210 discloses a separator for nonaqueous batteries. In this separator, an electrolyte solution having high ion conductivity is impregnated in open pores formed on two surfaces of a membrane electrolyte. As a result, ion conductivity of batteries is improved and the batteries maintain high current densities.
However, the areas of the positive-electrode sheet and the negative-electrode sheet must be increased in order to increase the discharge capacity of the lithium ion polymer secondary batteries. If the areas of these sheets are simply increased, the resulting batteries have large areas compared to the thicknesses thereof and will not be readily used. When the sheets are folded to solve such a problem, deflection occurs between the positive-electrode sheet and the negative-electrode sheet at folded portions, so that these sheets become detached from the electrolyte layer. Thus, the effective surface area of the interface between the electrodes and the electrolyte is reduced, resulting in a decreased discharge capacity and deterioration of discharge capacity characteristics after a number of discharge-charge cycles due to increased internal resistance. When the deflection is significant, direct contact between the positive-electrode sheet and the negative-electrode sheet, so-called xe2x80x9cinternal short-circuitingxe2x80x9d will occur at the deflected portion.
Since the polymer electrolyte layer disposed between the two active materials is relatively thin, these two active materials or collector foils, which are laminated at ends of the polymer electrolyte layer, may come into contact with each other by misalignment of lamination or by an external force applied to the laminate, resulting in internal short-circuiting.
As described above, the separator for nonaqueous batteries has open pores on the two surfaces thereof. If the separator insufficiently comes into contact with the positive-electrode sheet or the negative-electrode sheet, the electrolyte solution impregnated in these pores may leak. Moreover, intercalate/deintercalate cycles of ions in the electrodes cause a change in volume, and thus a gap may be formed between the separator and the positive- and/or negative-electrode sheets. Such a gap also causes leakage of the electrolyte solution. Because the sheet battery is bent according to the shape of the space for the battery in some cases, a gap may be formed between the between the separator and the positive- and/or negative-electrode sheets due to the stress during bending, resulting in leakage of the electrolyte solution. In addition, gas is produced in the battery during the charging/discharging cycles. When the gas is trapped on the surfaces of the positive and/or negative electrodes, the gas precludes ion mobility in the battery. Thus, the effective surface area at the interface between the electrodes and the electrolyte decreases, resulting in increased internal resistance and deterioration of discharge capacity characteristics after a number of discharge-charge cycles.
Accordingly, it is an object of the present invention to provide a lithium ion polymer secondary battery which does not cause internal short-circuiting, and which has a large discharge capacity and improved discharge capacity characteristics after a number of discharge-charge cycles.
It is another object of the present invention to provide a lithium ion polymer secondary battery which does not cause internal short-circuiting without a decreased discharge capacity.
It is still another object of the present invention to provide a gelatinous polymer electrolyte for sheet batteries, which is free from leakage of an electrolyte solution, which has improved discharge capacity characteristics after a number of discharge-charge cycles, and which exhibits high ion conductivity.
According to a first aspect of the present invention, a lithium ion polymer secondary battery includes a laminate of a strip of positive-electrode sheet having a positive-electrode collector foil and a first active material provided on the positive-electrode collector foil, a plurality of negative-electrode sheets, each including a negative-electrode collector foil and a second active material provided on the negative-electrode collector foil, and at least one polymer electrolyte layer. The polymer electrolyte layer is provided on at least one surface of the first active material, the strip of positive-electrode sheet is fan-folded at least one time, each of the negative-electrode sheets has a predetermined area corresponding to the area of flat portions of the folded positive-electrode sheet and is interposed between the flat portions of the folded positive-electrode sheet, and the polymer electrolyte layer is interposed between the first active material and the second active material.
According to a second aspect of the present invention, a lithium ion polymer secondary battery includes a laminate of a strip of negative-electrode sheet having a negative-electrode collector foil and a second active material provided on the negative-electrode collector foil, a plurality of positive-electrode sheets, each including a positive-electrode collector foil and a first active material provided on the positive-electrode collector foil, and at least one polymer electrolyte layer. The polymer electrolyte layer is provided on at least one surface of the second active material, the strip of negative-electrode sheet is fan-folded at least one time, each of the positive-electrode sheets has a predetermined area corresponding to the area of flat portions of the folded negative-electrode sheet and is interposed between the flat portions of the folded negative-electrode sheet, and the polymer electrolyte layer is interposed between the first active material and the second active material.
In the first or the second aspect, the strip of positive-electrode sheet or the strip of negative-electrode sheet is fan-folded. Thus, the lithium ion polymer secondary battery has a large discharge capacity without an increased area. Since the negative-electrode sheets or positive-electrode sheets are not arranged at folds of the strip electrode sheet, no deflection occurs between the positive-electrode sheet and the negative-electrode sheet. Since the first or second active materials on the separated positive- or negative-electrode collector foils are in contact with the same electrolyte layer, the internal resistance becomes uniform with respect to the active materials, resulting in improved discharge capacity characteristics after a number of discharge-charge cycles.
In the first aspect, the polymer electrolyte layer is preferably provided on at least one surface of the second active material. In the second aspect, the polymer electrolyte layer is preferably provided on at least one surface of the first active material. In such configurations, the polymer electrolyte layer contributes to a reduced internal resistance, improved discharge capacity characteristics after a number of discharge-charge cycles, and improved charge-discharge efficiency.
Preferably, in the first and second aspects, the polymer electrolyte layer covers the entire first active material so as to extend over at least one side edge of the first active material and/or the polymer electrolyte layer covers the entire second active material so as to protrude from at least one side edge of the second active material. A large contact area is ensured between the active materials and the polymer electrolyte layer. Thus, the battery has a large effective electrode area which contributes to reduced internal resistance. Moreover, the polymer electrolyte layer protects the active materials from drying. Thus, an increase in the internal resistance is suppressed and discharge capacity characteristics after a number of discharge-charge cycles and the charge-discharge efficiency are further improved.
Preferably, in the first aspect, one side edge of the positive-electrode collector foil protrudes from one side edge of each of the negative-electrode collector foils and the other side edge of each of the negative-electrode collector foils protrudes from the other side edge of the strip positive-electrode collector foil, the protruding portions of the positive-electrode collector foil are connected to a positive-electrode terminal, and the protruding portions of the negative-electrode collector foils are connected to a negative-electrode terminal. Preferably, in the second aspect, one side edge of the negative-electrode collector foil protrudes from one side edge of each of the positive-electrode collector foils and the other side edge of each of the positive-electrode collector foils protrudes from the other side edge of the negative-electrode collector foil, the protruding portions of the negative-electrode collector foil are connected to a negative-electrode terminal, and the protruding portions of the positive-electrode collector foils are connected to a positive-electrode terminal.
In the above configurations, the positive-electrode terminal and the negative-electrode terminal can be readily provided in the lithium ion polymer secondary battery.
According to a third aspect of the present invention, a lithium ion polymer secondary battery includes at least one positive-electrode collector foil provided with a first active material on a surface thereof, at least one negative-electrode collector foil provided with a second active material on a surface thereof, and at least one polymer electrolyte layer. The positive-electrode collector foil, the polymer electrolyte layer, and the negative-electrode collector foil are laminated so that one side edge of the positive-electrode collector foil protrudes from one side edge of the negative-electrode collector foil and the other side edge of the negative-electrode collector foil protrudes from the other side edge of the positive-electrode collector foil. Insulating films are provided in both side edges of the polymer electrolyte layer over the entire length so as to protrude from the side edges.
In this lithium ion polymer secondary battery, the insulating films provided at both edges of the polymer electrolyte layers protect the positive-electrode sheets and the negative-electrode sheets from short-circuiting due to misalignment in the lamination process or external force applied to the laminate. If the edges of the polymer electrolyte layer are melted in the thermal compression bonding process, the insulating films protect the positive-electrode collector foil and the negative-electrode collector foil from short-circuiting due to the melt of the polymer electrolyte layer.
Since each polymer electrolyte layer satisfactorily functions even at the side edges having the insulating films, the effective areas of each positive-electrode sheet and each negative-electrode sheet do not decrease.
Preferably, a ratio t2/t1 of a thickness t2 of the insulating film to a thickness t1 of the polymer electrolyte layer is in a range of 0.01 to 0.7. In such a ratio, the insulating films do not cause an increased total thickness of the polymer electrolyte layers. Thus, the lithium ion polymer secondary battery can be prevented from increasing in size due to the insulating films.
Preferably, a ratio s2/s1 of a length s2 of the protruding portion of the insulating film at the side edge of the polymer electrolyte layer to a length s1 of the protruding portions of the negative-electrode collector foil and the positive-electrode collector foil is preferably in a range of 0.02 to 0.8.
Since the ratio s2/s1 is in the specified range, the insulating film can securely prevent contact between the active materials and between the positive-electrode collector foil and the active material. Thus, internal short-circuiting between the positive-electrode sheets and the negative-electrode sheets in the laminate does not occur and the discharge capacity can be readily increased.
According to a fourth aspect of the present invention, a gelatinous polymer electrolyte, interposed between a positive-electrode sheet and a negative-electrode sheet of a sheet battery, includes a gelatinous polymer, wherein many closed pores are substantially uniformly confined in a matrix of the gelatinous polymer and are filled with at least gas and optionally an electrolyte solution.
The gas in the closed pores moderates a change in volume and internal pressure due to discharge and charge of ions in the electrodes and a stress generated when the sheet battery is bent. Thus, the polymer electrolyte does not separate from the positive-electrode sheet or the negative-electrode sheet due to external force. As a result, the gelatinous polymer electrolyte improves discharge capacity characteristics after a number of discharge-charge cycles of the battery compared to conventional solid polymer electrolyte.
Preferably, the pores in the gelatinous polymer electrolyte have diameters of 5 to 20 xcexcm and occupy 0.1 to 30 percent by volume of the matrix of the gelatinous polymer. The discharge capacity characteristics after a number of discharge-charge cycles are further improved.
Preferably, the pores are filled with 0 to 30 percent by volume of the electrolyte solution and 70 to 100 percent by volume of the gas.
In the gelatinous polymer electrolyte having such a configuration, the electrolyte solution is trapped in the pores and contributes to ion conduction. Thus, the gelatinous polymer electrolyte exhibits improved ion conductivity compared to conventional solid polymer electrolytes, resulting in reduced internal resistance of the battery. The battery exhibits sufficient functions even if the electrolyte solution is not trapped in the pores.